The present invention relates to a tuned oscillator utilizing a thin film ferromagnetic resonator and a microwave communication apparatus which uses the tuned oscillator.
Conventional local oscillators used in the RF stage of microwave communication equipment include oscillators which use dielectric resonators (DRO) and voltage controlled oscillators (VCO). The DRO, owing to a high Q value of the dielectric resonator, produces low phase noise, which allows data communication equipment to operate at a low bit error rate (BER) and video signal communication equipment to operate at a large S/N ratio.
Since it is a fixed oscillator, the DRO bases its operation on block conversion, in which signals in the communication band are converted in batchs into the intermediate frequency (IF) and a necessary signal is selected with the VCO in the IF stage, and which causes a problem because an intricate IF stage which includes a tracking filter for blocking the Image signal is required. In case of using a VCO as a local oscillator, signals can be selected in the RF stage, which simplifies the IF stage, but due to a low Q value of the varactor diode which is used as an oscillation active element the phase noise characteristics are inferior, which causes in a deteriorated BER characteristics for data communication and deteriorated S/N ratio for video signal communication.
Recently, there has been proposed a tuning oscillator which utilizes the ferrimagnetic resonance of a yttrium iron garnet (YIG) sphere which is a ferrimagnetic material (Japanese Patent Publicatin No. 53-32671). This YIG-tuned oscillator (YTO) has a high Q value for the YIG resonator and produces low phase noise and its use of magnetic resonance provides a satisfactory linear tuning characteristics, which are advantages for both DRO and VCO devices.
Preceding publications pertinent to the present invention include U.S. Pat. No. 4,626,800 which describes a tuned oscillator which uses an YIG thin film magnetic resonator, and U.S. Pat. Nos. 4,745,380 and 4,746,884 for the technique of compensating the temperature dependency of a ferrimagnetic resonance frequency of the thin film YIG resonator by replacing part of the iron ion (Fe.sup.3+) of the YIG thin film with a non-magnetic ion.
However, the conventional YTO which uses the above-mentioned YIG sphere has been difficult to use as a local oscillator for communication equipment for the following reasons. Primarly, the d.c. magnetic field which is applied to the YIG sphere depends on the intended oscillation frequency which is generated entirely by feeding current to the coil, which not only results in a large current consumption in the coil, but a large inductance of the coil due to its large number of turns which slows the turning and requires larger dimensions for the oscillator. Secondarily, the difficulty in making YIG sphere results in YTOs with YIG spheres to be unfavorable for mass production and also expensive, and accordingly it is undesirable for the communication equipment in which mass-production and low cost are required.